Video tape recorders having two rotary magnetic heads separated by 180.degree. which record a video signal in alternate successive slant tracks on a magnetic tape are well known in the art. Recently, these video tape recorders (VTRs) have been provided in an 8 mm format in which the rotary heads record both the video signal and an accompanying digital audio pulse code modulated (PCM) signal in separate record sections of each of the successive slant tracks. One such VTR is disclosed in U.S. Pat. No. 4,551,771, issued Nov. 5, 1985 and assigned in common with the present application. As disclosed therein, the tape is wrapped around a rotary drum mounting the rotary heads in an arc greater than 180.degree., so that each track scanned by a head includes an overscan section subtending a small angle and then a main section subtending 180.degree.. During recording, the audio signal associated with each field of the video signal is converted into a block of digital audio PCM words, error encoded and time base compressed so that it may be recorded within the overscan section of one track. The time required for the conversion, encoding and time base compression generally corresponds to one field interval of the accompanying video signal, so that the block of audio PCM data is available for recording only after the corresponding field of the video signal has been recorded, and is therefore recorded in the overscan section of a following track. The audio PCM signal is supplied to the rotary heads while they trace the overscan sections and the video signal is supplied to the rotary heads while they trace the main sections so that the audio and video signal are recorded in the overscan and main sections, respectively.
During reproduction, the slant tracks are alternately traced by the two rotary heads to produce output signals. The digital audio PCM signals are time base expanded, decoded and converted into analog form, a process which also requires a time corresponding to one field interval of the video signal. Consequently, a total delay between the audio signal and the corresponding video signal created by recording and subsequent reproduction amounts to two field intervals.
One highly advantageous method of error correction encoding which may be applied to the audio PCM data recorded by an 8 mm VTR is the cross interleaving method disclosed in U.S. Pat. No. 4,562,578, issued Dec. 31, 1985 and assigned in common with the present application, in which index words are annexed to blocks of digital audio words obtained by digitizing fields of stereo audio signals, and the combined words are then scrambled, parity and error correction codes are added thereto and the resulting data is converted into serial data for transmission. This method provides the ability to correct errors in the recorded digital audio signals to a very high degree of accuracy.
However, the cross interleaving method requires that the index words, error correction words etc. be thoroughly intermixed with the audio data words, with the result that it is impossible to edit or dub a limited area within each block recorded in each overscan section while leaving the remaining areas unchanged. In fact, in order to edit any of the audio signal, it is necessary to reproduce the entire block, mix or dub the audio data, recompute the error correction codes and rerecord the entire new block. Since the time base compression and expansion, conversion and error correction processing require a total delay of two field intervals, that is, one field interval during reproduction and one field interval during recording, each dubbing operation will cause each block of audio signal to drop two more field intervals, i.e. two more tracks, behind its corresponding field of video signal. As a result, the timing control required for presenting the audio signal simultaneously with the video signal becomes extremely complicated.